I’ve been using Grangers 2in1 washer and proofer consistently (must be 8-10 washes now) on a Gill jacket that was previously washed using detergent, obviously I know now how stupid this was, but it doesn’t seem to be improving the jacket’s DWR properties.

Is there anything else I can try or is time to give it up and buy a new jacket?

tumble dried it? or iron it on the lowest setting

Try Nikwax Tech Cleaner. That should strip off any detergent residues. Than re-proof. Check the washing instructions to see if you can tumble dry or warm iron.